A powerful technology for analyzing matter involves measuring the reflectance from or the transmission through the matter to be analyzed at a narrow band wavelength in the near infrared region known as NIR. To carry out such analysis, the matter is irradiated with NIR light and the amount of light transmitted through or reflected from the matter at narrow band wavelengths is measured and analyzed. Instruments for making such analyses are disclosed in U.S. Pat. No. 4,969,739 to Phillip A. McGee, U.S. Pat. Nos. 4,264,205 and 4,285,596 to Issac J. Landa and U.S. Pat. No. 4,040,747 to Donald R. Webster. Frequently, such infrared analyzing instruments employ a fiber optic probe as a convenient technique to transmit the light from a near infrared source to the sample to be analyzed and to receive diffusely reflected light back from the sample and transmit the received light back to the instrument. A window is employed in the distal end of the probe between the transmitting fibers and the sample, which window must be transmissive to infrared light. Typically, the window is made of sapphire or quartz and has a relatively high index of refraction. The surfaces of the window, and particularly the outer surface of the window being a plane optical surface at which the index of refraction changes abruptly, has a tendency to specularly reflect light and it is important to prevent this specularly reflected light from being received by the receiving optical fibers in the fiber optic probe and transmit it back to the instrument as such specularly reflected light intertwines with the instrument capability of detecting the intensity diffusely reflected light from the sample.
U.S. Pat. No. 5,166,756, issued Nov. 24, 1992 to Phillip McGee et al., discloses a fiber optic probe which achieves elimination of spectral reflection by forming the outer face of a window block of the probe skewed at an angle to the axis of the probe. Pending U.S. application Ser. No. 07/931,783 discloses a probe with a two-piece window with a window block having a skewed internal face to prevent spectral reflection from being received.